Repetitive exercise has been found to produce an increase in the size of the coronary vascular bed. The enlarged vascular capacity may furnish protection to the heart that is further stressed by a pressure overload by aortic constriction. Pressure overload will stimulate a cardiac enlargement that has been shown to result in heart failure or near failure. The physiological, biochemical, and ultrastructural changes in repetitive exercise followed by pressure overload will be studied in the chronic instrumented dog. The dogs will be studied during control periods and at intervals following aortic constriction. Physiological data will be obtained from the intact animal during an exercise test, cardiac pacing, and short coronary artery occlusions. The biochemical function of the mitochondria, sarcoplasmic reticulum, lysosome, and myofibrillar ATPase will be studied at the conclusion of the experiments in these same animals. The ultrastructural changes in the cardiac muscle will be correlated with the biochemical and physiological responses. The transmission and scanning electron microscope will be used to determine cell membrane to myofibril ratio changes and changes in surface characteristics of the muscle cell. Any beneficial effect of repetitive exercise should change the relationship between the degree of enlargement and functional capacity of the heart.